Optical amplifiers are important components of fiber-optic communication systems. Erbium-doped fiber amplifier (EDFA) systems have become especially popular owing to their gain characteristics near the 1.5 μm transmission band of conventional optical fiber. An example of a typical conventional EDFA 10 is shown in FIG. 1. It is to be kept in mind that FIG. 1 is provided for example only and that, since EDFA designs are varied, not every component shown in FIG. 1 may be included in a particular EDFA design. Alternatively, more-complex EDFA designs may include additional components not shown in FIG. 1. In FIG. 1, an optical signal, typically comprising one or more wavelengths within the range of about 1527-1565 nm, is input through a first fiber 12a to a first optical coupler 14a. The optical coupler 14a delivers most of the power of the optical signal to a second optical fiber 12b but separates a small proportion (ca. 1-5%) of the original optical power to a third fiber 12c which leads to a first photodetector 15a. The main signal proportion passes through a first optical isolator 16a, which prevents amplified light and pump laser light produced within the amplifier 10 from propagating in a reverse direction to and within the first fiber 12a. The signal passing through the first isolator 16a is then delivered through fourth fiber 12d to a first multiplexer/de-multiplexer (MUX/DEMUX) 18a where it is combined with a first or co-propagating laser light (typically either near 980 nm or near 1480 nm) produced by first pump laser 22a and delivered to the MUX/DEMUX through fifth fiber 12e. 
An Erbium-doped fiber 20 within the amplifier 10 (FIG. 1) receives both the signal light and the co-propagating laser pump light from the first MUX/DEMUX 18a. Further, the Erbium-doped fiber 20 receives a second or counter-propagating laser pump light (typically either 980 nm or 1480 nm) produced by a second pump laser 22b and delivered to the Erbium-doped fiber 20 by sixth fiber 12f and second MUX/DEMUX 18b. The co-propagating and counter-propagating pump lights travel through Erbium-doped fiber 20, respectively, in the same direction as and in the opposite direction to the optical signal.
The optical signal is amplified within the Erbium-doped fiber 20 (FIG. 1) as a result of stimulated emission caused by the pumping of Erbium-ion electrons under the combined effects of optical pumping by the two laser pump lights. The amplified optical signal is separated from the second pump light by the second MUX/DEMUX 18b and is delivered to the second optical isolator 16b through the seventh fiber 12g. The second optical isolator 16b prevents any reflected signal light from being inadvertently input to the Er-doped fiber where it would be amplified and possibly contaminate the signal light. After passing through the second isolator 16b, the amplified signal light passes to second optical coupler 14b via the eighth fiber 12h. The second optical coupler 14b delivers most of the optical power of the amplified signal light to the ninth or output fiber 12i. However, the second coupler also removes a small sample proportion of the amplified optical signal to a second photodetector 15b via a tenth fiber 12j and removes a small sample proportion of any reflected signal light to third photodetector 15c via an eleventh fiber 12k. 
Each of the photodetectors 15a-15c within the EDFA 10 produces an electrical signal that is proportional to or in relation to the optical power of the sample light received by the respective photodetector. The electrical signals produced by the photodetectors 15a-15c are delivered to a control module 24 via electrical lines 17a-17c, respectively. The control module 24 monitors the amplifier system performance based upon these input electrical signals and optimizes the overall amplifier performance by sending control signals to the pump lasers 22a-22b via the electrical lines 17d-17e, respectively.
Since physical space within an optical amplifier installation may be severely limited, the many components comprising the EDFA 10 may be arranged in close proximity to one another in a configuration designed to make the best use of all available space. FIG. 2 shows an example of a housing configuration for an EDFA. The optical and electronic components of the EDFA are housed within a container 30 that contains an internal spool 32 upon which the various fibers are wound. The relatively bulky pump lasers 22a-22b and photodetectors 15a-15c are generally mounted upon a printed circuit board secured to the inside of a wall of the container 30 and the various “pigtail” fibers 12c, 12e, 12f, 12j, 12k that connect to these lasers and photodetectors emerge tangentially from the fiber windings around the spool 32. The Er-doped fiber as well as the remaining isolator, coupler, MUX/DEMUX, remaining fiber lengths and any splices between these components, none of which are explicitly shown on FIG. 2 but which are assumed to be present, are housed on or within the spool 32.
The close proximity of components within the conventional optical amplifier configuration (FIGS. 1-2) presents some problems in terms of unwanted light transfer between components. For instance, when viewed with an IR viewer, the inventor has noted that stray pump laser light “leaks” from the system at a number of different locations, including near the pump housing and in the vicinity of splices and couplings between fibers or between fibers and other components. By far the largest power flux is from the initial coupling of a pump laser 22a-22b into the fiber pigtail 12e-12f to which it is directly coupled.
The unwanted and leaking pump light primarily resides within very loosely bound cladding modes within the various fibers. The term “cladding mode” as used herein, is not meant to be limited to light propagating exclusively within the cladding but may also include light that propagates within other components of the optical fiber—such as protective acrylate coatings—in addition to the cladding. This cladding mode propagation arises because the coupling from the pump lasers 22a-22b into the cores of the pump laser pigtail fibers 12e-12f is not 100% efficient and a significant proportion of the laser power is launched into the cladding and coating of the fibers. Some of this power is not in a truly guided mode and the cladding and coating are acting more like a lossy “light pipe”. The propagation of signal light and stray pump laser light within a pigtail fiber 12 is illustrated in greater detail in FIG. 3. The fiber 12 shown in FIG. 3 comprises a conventional optical fiber having a core 46 surrounded by a cladding 44. A first light 48 propagates in one or more conventional guided modes within the core 46. A second light 49 propagates in loosely bound cladding modes within both the core 46 and cladding 44. If the pigtail fiber 12 shown in FIG. 3 is one of the pump laser pigtail fibers 12e-12f, then the first light 48 and the second light 49 comprise the same wavelength, generally around 980 nm or 1480 mm. If the pigtail fiber 12 of FIG. 3 is one of the other fibers, such as one of the fibers 12c, 12j, 12k that couple directly to the photodetectors, then the second light will be at the wavelength emitted by a pump laser (980 nm or 1480 nm) and the first light 48 will comprise a wavelength utilized for optical signal transmission, generally within the well-known “C” band ranging from about 1527-1565 nm.
Within the pigtail fiber 12 (FIG. 3), the first light 48 is constrained to propagate near the core, constrained by wave guide principles. However, a proportion 49a of the second light 49 may exit through the cladding air interface and thereby exit the fiber 12. This light 49a may then be available to enter the cladding of any other fiber that may be adjacent to the fiber 12, in a fashion that is just the reverse of the light loss phenomenon shown in FIG. 3. Although a bare optical fiber is indicated in FIG. 3, the fiber may have an outer coating surrounding the cladding 44, such as those that are commonly used to protect the fiber from mechanical breakage or chemical attack. In such a case, the second light may also propagate within loosely bound modes that include the coating, since such coatings are generally transparent to infrared light. The leakage of pump laser light illustrated in FIG. 3 presents a difficulty and potential disadvantage regardless of whether the fiber 12 is coated or uncoated.
Thus, when observed with an IR viewer, various pigtail fibers appear to “glow” over a length of several centimeters, indicating that light is being emitted outward from cladding modes over such a length. If another piece of fiber is near this “glowing” fiber, some of the light is coupled into this second fiber's cladding or coating and can propagate therein for many centimeters. If this second fiber happens to be the pigtail from one of the monitor photodiodes 15a-15c, then, when the pump is turned on, there is a coupling of pump light into this monitor photodiode. Since the pump laser light can be orders of magnitude higher in power in comparison to the optical signals that the monitor photodiodes are designed to detect, even a small amount of such leakage can perturb the photodiode signals and, as a result, the operation of the EDFA as a whole.
Complicating the pump laser leakage problem noted above is the fact that many EDFA's are dual-stage or multi-stage amplifiers comprising additional pump lasers and monitor photodiodes and having additional components, such as gain-flattening filters or mid-stage access ports. The resulting duplication of components and fibers (relative to those shown in FIG. 1) and the provision of additional components as well as the splices or couplers between such components causes even more opportunities for light cross contamination within the housing configuration shown in FIG. 2.
Conventional means of solving the pump laser light leakage and contamination problem noted above include choosing fiber components having a coating or covering thereupon, prior to assembling the amplifier, in order to prevent the cross-fiber coupling or else mechanically separating the various fibers that may be subject to inter-fiber coupling cross contamination. Either of these methods can adequately remove the unwanted laser pump contamination light before it reaches the photodetectors.
The use of a cover or coating on the fiber (so as to prevent the coupling) disadvantageously requires that very long sections of fiber be so covered or coated in order to assure that no coupling takes place within any section of any of the various fibers. Most of such covering or coating will be unnecessary since potential inter-fiber coupling only occurs at certain locations. Further, if any inter-fiber optical coupling should occur at some point because of, for instance, a break in the covering or coating, then the remaining covering or coating will be rendered useless since it cannot remove the contamination light once it has entered a fiber.
Finally mechanical separation of fibers adds significant additional bulk to the overall apparatus.